A chemical vapor deposition (CVD) process is a deposition process where reactants are flowing together into a reactor chamber. CVD relies upon gas flows that are fully developed and tightly controlled to avoid fluctuations in pressures or gas flow rates/profiles. A number of CVD techniques have been developed, e.g., atmospheric pressure CVD, low pressure CVD and ultra-high vacuum CVD. Variations of the different pressure regime CVD processes include variations relating to methods of heating the chamber or substrate being processed. Many particular regimes of CVD are known to artisans. In general, CVD provides fast growth rates for thick layers and produces high quality layer when pressures, precursor ratios and temperatures are precisely maintained.
Atomic layer deposition (ALD) is a usually a slower process than CVD and is useful for forming thin films of a few nanometers. ALD depends upon sequential or pulsed introduction of precursors that react on a substrate. Between each precursor pulse in ALD, the reaction chamber is purged to remove precursors that did not chemisorb/react and reaction by-products. Each reaction cycle adds a given amount of material to the surface, referred to as the growth per cycle. The process is very slow because only a fraction of a monolayer is typically deposited during each reaction cycle.
Generally, conventional deposition systems and methods form either CVD or ALD layers. Devices formed from a combination of the two processes require two systems and a break of vacuum. As is known, the surface chemistry of layers can therefore be negatively affected.
Published U.S. patent application 20080317972 describes an ALD system that can operate in a “pulsed CVD mode”. Such a pulsed mode produces pressure variations and layers that are realized by adjusting pulse lengths of the precursors or the purge gas. This pulsed operation obtains Si-rich HfSiO films that are difficult to obtain by conventional ALD. The pulsed CVD operation is likely to be a much slower process than continuous or true CVD and would likely produce lower quality films. The process is suitable for HfSiO and HfSiON films for which the desired thicknesses are typically of the order of a few nanometers. However, growth of thick multi-component and multilayered structures would be highly impractical by this “pulsed CVD” method.
Complex thin multi-metal oxide films have a number of promising applications. Thin epitaxial ferrite films of CoFe2O4, NiFe2O4 and (Mn,Zn)Fe2O4 deposited using pulsed laser deposition and sputtering have been studied because of their potential use in exchange-coupled biasing based devices. More recently, BiFeO3 has generated interest due to its room temperature multiferroic properties. Heterostructures of ferrite and piezoelectric materials have great potential in magnetoelectric devices. Complex metal oxide film structures are also important in applications such as thermoelectric devices and solid oxide fuel cells. In order to deposit such complex oxide structures for fabrication of devices at a commercially viable scale, metalorganic chemical vapor deposition (MOCVD) is one of the most suitable techniques.
Iron-based perovskite materials have been developed as promising materials for solid oxide fuel cell cathodes and photocatalysis. See, e.g., Ralph et al., US Published App. US2005106447. Thin epitaxial ferrite films of CoFe2O4, NiFe2O4 and (Mn,Zn)Fe2O4 deposited using pulsed laser deposition and sputtering have been recently studied because of their potential use in exchange-coupled biasing based devices. Y. Suzuki, R. B. van Dover, E. M. Gyorgy, J. M. Phillips, V. Korenivski, D. J. Werder, C. H. Chen, R. J. Felder, R. J. Cava, J. J. Krajewski and J. W. F. Peck, J. Appl. Phys., 79, 5923 (1996). More recently, iron-based perovskite BiFeO3 has generated a lot of interest due to its room temperature multiferroic properties that could lead to the development of novel devices. J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D. G. Schlom, U. V. Waghmare, N. A. Spaldin, K. M. Rabe, M. Wuttig and R. Ramesh, Science, 299, 1719 (2003). For deposition of such complex oxide structures for fabrication of devices at a commercially viable scale, metalorganic chemical vapor deposition (MOCVD) is one of the most suitable techniques. However, applicability of this deposition technique is largely dependent on the availability of suitable liquid precursors.
MOCVD can deposit conformal thin films over large areas with the right stoichiometry, high yield and high throughput at low cost. Precursors should have a stable and reproducible high enough vapor pressure, should not decompose during transport to the reactor and should not be hazardous. Gas or liquid precursors with suitably high vapor pressures are ideal for MOCVD because of the ease with which they can be delivered to the deposition chamber in a controlled manner. For gases, a mass flow controller is conventionally used to control the dose of precursor from the precursor cylinder. In case of liquids, the use of bubblers is the most dominant delivery method. A carrier gas at a controlled flow rate is fed through the liquid in the bubbler so as to entrain the precursor molecules, thereby delivering the precursor to the reactor. The bubbler is maintained at a specific temperature using a temperature controller in order to achieve a constant equilibrium precursor vapor pressure. Thus, in both cases, a simple and inexpensive arrangement is needed to reproducibly control the vapor phase concentration of the precursor in the reactor. On the other hand, solid precursors pose several problems that may limit their suitability for MOCVD. Solid precursors have relatively low vapor pressures compared to liquids. More importantly, their vapor pressures decrease because of changing surface area as they are consumed. Also, lower vapor pressures require that solid precursors be used at elevated temperature to increase the vapor pressure; but this could lead to agglomeration and/or degradation of the precursor in the precursor vessel. In order to overcome these difficulties with solid precursors, complex and expensive non-conventional delivery approaches, like using a separate heating zone within the reaction chamber, solvent-based direct liquid injection and sublimation in a fluidized bed, have been employed. While such solid precursor delivery techniques provide possible solutions, they also bring in additional parameters that increase the complexity of the MOCVD process compared to the use of liquid precursor delivery schemes.
Iron precursors belonging to mainly four classes have been reported for the chemical vapor deposition of different iron-containing oxide films: (i) carbonyls, (ii) β-diketonates, (iii) metallocenes and (iv) alkoxides. Iron pentacarbonyl (Fe(CO)5) is a liquid precursor that has been used with different carrier gases to deposit iron and iron oxide films, but its vapor pressure is too high (˜28 Torr at 25° C.) to easily control its delivery to the reaction chamber in the deposition of thin films for electronic applications. Furthermore, apart from being toxic and pyrophoric, it has a low decomposition temperature (˜180° C.), which could be a limitation for its use in the deposition of crystalline iron oxide films requiring high deposition temperatures. All the other iron precursors used for CVD are in the form of crystalline powder at room temperature. β-diketonate-based iron precursors have been the most widely reported sources for depositing iron oxide films: iron (III) acetylacetonate (Fe(acac)3), ferric dipivaloyl methanate (Fe(DPM)3), iron(III) t-butylacetoacetate (Fe(tbob)3) and tris(trifluoroacetylacetonato) iron (III). Fe(DPM)3 is also known as iron(III) tris(2,2,6,6-tetramethyl-3,5-heptanedionate) (Fe(tmhd)3 or Fe(thd)3) and its use for atomic layer deposition of iron oxide films has been reported recently. Besides being solid at room temperature, the β-diketonates suffer from potentially serious disadvantages; they undergo degradation easily in the delivery lines as well as under storage in the precursor vessel. Recently, solid iron tert-butoxide ([Fe(OtBu)3]2), which belongs to the alkoxide family, has been used as a single source precursor. More recently, Fe(II) dihydride complexes H2Fe[P(OCH3)3]4 and H2Fe[P(CH3)3]4 have been reported for MOCVD of iron oxide films. These precursors are not available commercially and, furthermore, H2Fe[P(OCH3)3]4 produced films that were amorphous and had phosphorus contamination. Among metallocenes, only Fe(C5H5)2 has been reported for MOCVD applications, to the knowledge of the inventors. Though it had several advantages over the other precursors mentioned earlier, being a solid precursor it required a use of fluidized bed evaporator for its delivery to the reactor.
α-Fe2O3 thin films have been deposited on Si(100) substrates using n-butylferrocene and oxygen in a low-pressure metalorganic chemical vapor deposition reactor. The iron precursor is liquid at room temperature having a high enough vapor pressure; its thermogravimetric analysis shows that it undergoes clean evaporation without decomposition. The growth rates were studied in the temperature range of 400-600° C. The resulting thin films were characterized for structure and morphology using X-ray diffraction and scanning electron microscopy. Their composition was analyzed using energy-dispersive X-ray spectroscopy and chemical bonding states were probed using X-ray photoelectron spectroscopy. Films deposited at 450° C. were mostly non-crystalline and had carbon contamination. Films deposited at higher temperatures were crystalline α-Fe2O3.
Bismuth ferrite (BiFeO3) is multiferroic, having a high ferroelectric Curie temperature and developing spiral-antiferromagnetic order below 643 K. It is unique because BiFeO3 (BFO) is the only known single-phase material that is multiferroic at room temperature. Therefore, it has enormous potential for applications in spintronic devices, non-volatile ferroelectric random access memory, microwave technology, sensors and microactuators. MOCVD is also a preferred technique for BFO.